In order to perform large capacity data communication over a short distance or a medium distance such as a distance between server boards or between network devices, optical interconnect technique using an optical transceiver module is being adopted. When a data rate higher than 200 Gbps (50 Gbps×4 lanes) as in 200 GBASE-DR4 standard is achieved, the data rate per lane exceeds 50 Gbps. It is difficult to achieve such mass transmission by non-return-to-zero (NRZ) scheme that performs two-level symbol transmission, and multi-level optical transmission such as 4-level pulse amplitude modulation (PAM-4) is called for.
In order to perform communication with PAM-4 signal over a distance of 500 m or longer, external modulation system in which an external modulator modulates light from a light source has been changed over from direct modulation system in which on and off of a laser represents an optical signal. As an external modulator, what draws attention is a small-size MZ modulator which is formed by using silicon photonics technology. In an MZ modulator, an optical coupler splits light from a light source into optical signals, and then the optical signals in upper and lower arms are provided with a phase difference by phase shifters of the upper and lower arms with application of an electrical signal thereto to produce output light with low or high intensity.
A silicon-based optical modulator has a configuration in which a modulator arm includes M separated segments, and N-bit input data signal is converted to multiple M drive signals for the M modulation segments (M≥2N/2).
As illustrated in FIG. 1, for an optical modulator using silicon photonics technology or an optical modulator formed by a semiconductor optical waveguide of another technique, a phase difference between arms varies non-linearly with an inputted electrical signal. In FIG. 1, the horizontal axis indicates input voltage, and the vertical axis indicates amount of phase shift (rad). For instance, when four levels are respectively represented by four voltage levels, the greater the input voltage is, the more deviation occurs from linear characteristics. Therefore, as illustrated in FIG. 2, the characteristics of optical output with respect to input voltage of an MZ modulator does not form a sine wave, and even if input voltage is controlled to rise and fall at regular intervals, opening of the eye pattern of optical signals is non-uniform due to amplitude distortion.
When each arm of the MZ modulator is provided with multiple segments to cope with the problem of non-linearity, loss increases according to the length of the segments. As an output of LD is increased in order to compensate the loss, power consumption increases. In addition, the power consumption also increases by driving the same number of drivers as the number of the segments.
The following is a reference document.
[Document 1] Japanese National Publication of International Patent Application No. 2013-502613.